This invention relates generally to electronic control circuitry and more particularly, it relates to an adjustable undervoltage trip fault interrupt circuit for preventing "nuisance" undervoltage fault trips caused by instantaneous sags in the AC power lines of less than a selectable time.
As is generally known, AC adjustable frequency drive controllers are typically provided for controlling the flow from centrifugal fans or pumps used in HVAC systems located in various industrial plants. The drive controller is used directly to accomplish this function by controlling the speed of an AC motor. Such an adjustable frequency drive controller is commercially available from the Graham Company of Milwaukee, Wis., under their Model Nos. 1576AFC15-300.
An undervoltage detection or sensing circuit is generally provided for the drive controller for rendering undervoltage protection. Upon the decrease or drop in the incoming line voltage below a set point, a trip signal will be generated by the undervoltage circuit to cause the shutting down of the drive controller and thus in turn causing the AC motor to stop. The major problem experienced is that the trip signal will be generated even when only a very short momentary sag occurs in the AC power lines. This is referred to as a "nuisance" fault trip which causes the drive controller to be turned off or shut down for a period of time. As a result, there is produced an undesired down time which reduces the efficient operation of the system.
Accordingly, it would be desirable to provide an adjustable undervoltage trip fault interrupt circuit for preventing "nuisance" undervoltage fault trips caused by instantaneous sags in the AC power lines of less than a selected time. It would be also expedient to provide the fault interrupt circuit with a variable time delay circuit in order to insure that the transient voltage drops of less than a predetermined amount of time will not produce the trip signal.